Temperature control of graphitemoderated nuclear reactors



Jlm,H 196573` s. FAwcETT ETAL 4 Sheets-Sheet l Original Filed Aug. 24,1959 Fi G. i

TEMPERATURECONTROL OF GRAPHITE-MODERATED NUCLEAR REACTORS 4 Sheets-Sheet2 Original Filed Aug.` 24, 1959 TEMPERATURE CONTROL OF'GRAPHITE-MODERATED NUCLEAR REACTORS rignalmFled Aug. 24; 1959 4Sheets-Sheet 5 f lloamo f-lzo Jn 3,* 1967 1 s. FAwcETT ETAL 3,296,084

TEMPERATURE CONTROL OF GRAPHITE-MODERATED NUCLEAR REACTORS OriginaLFiledAug. 24, 1959 4 Sheets-Sheet 4 |20 E22 ISS `negative `:reactivitycoefficient.

3 296,084 TEMPERATURE CNTROL `F GRAPHITE- MODERATED NUCLEAR REACTORS l`Sydney Fawcett, Hale Barns, Richard Valentine Moore,

Appleton, ;and t Burton Cutts, Culcheth, Warrington, England,ljassignors. to UnitedKingdom Atomic Energy Authority Continuation rofapplication SenNo. `835,523, Aug. 24, 19.59.` This application Jan. 22,1965, Ser. No4-127,303

1 Claims priority,.applicationtGreat Britain, Sept. 4, 1958,

28,499/58 Claims. (Cl. 176-59.)

V This invention relates to nuclear reactors and is a continuation of`our application Serial No. 835,523 filed August 24, ,1959, nowabandoned. The invention is concerned With `terrlperaturecontrol ofgraphite moderated i nuclear` reactors.

` Ajgas-cooled, natural or only slightly enriched uraniumfuelledgraphite moderated `reactor has a reactivity depende`r1t1;upon `thetemperature of components in the refactor.

Where reactivity increases `with temperature increase the reactor issaid to have positive reactivity coeicienti (expressed in units ofreactivity change per degreezcentrigradeqtemperature change) and whereit decreases: with temperature, a negative reactivity coefficient. The`positive coefficient represents a tendency to instability which is notnecessarily serious in itself so long as its respouse timeis long enoughto allow control.

Thea fuel. in the` reactor has, considered on its own, a When consideredwith graphite however, conditions can occur that give rise to A`c`verall1positive coefficient. These conditions arise as the u plutonium`content of the fuel increase, plutonium having therproperty (inthetemperature ranges `being considered) of an increasing fissioncross-,section as neutron energies increase (i.e`., as moderatortemperatures increase). If the increased fission rate is leftuncontrolled a further temperatureincrease in the graphite takes placeafter a short ther- .j mal time lag, :and the fission rate is increasedfurther. lThis temperature sensitive growth of fission rate can, of

course, be arrested by the operator but it may be consideredinadvisableor even impracticalto burden the opera- Utor `withthercontrol ofcontinuously changing factors of `which that referred to above is onlyone.

Itfisp therefore, anfobject of the present invention to provide.arrangements in a graphite moderated nuclear reactor removing `ormitigating a positive reactivity coeliieient `According to theinvention, a graphite moderated nu- "`cl`eara1reactory has means forconfining at least a portion lof the coolant entering the reactor topassage through the graphite moderator prior to its contra-directionalpassage in contact with fuel `elements orientated in channels in `themoderator, thereby cooling the moderator to reduce the amountjof itstemperature variation and bring about j reduction of theeifects ofpositive moderator temperature coefficient.

Where only a portion of the coolant entering the reactor moderator,andtherotherfraction being constrained to `ow in contact with themoderator along an annular passage, in each fuel element channel, `whichpassage is separatedzfrom the passage for fuel element-contactingcoolant ineach channel. The two fractions may be combined for`ccuitra-directional flow together in contact with the fuel a united;states Parent O drawings, wherein:

ice

elements. Alternatively, the coolant may be divided into two fractionsafter having passed through the moderator and one fraction may beconfined to flow along said annular passage whilst the other fraction isconstrained to iiow over the fuel elements.

The said annular passage in each fuel element channel may be outsidesleeving means disposed betwee-n the fuel elements and the wall of therespective channel and spaced from the said wall to form an annulus.

In order to reduce heat degradation by heat transfer from the fuelelements to the moderator coolant fraction in the said annulus, athermal barrier is Vpreferably disposed between the fuel elements andthe said sleeving means in each channel, whereby thermal insulation ofsaid annular passage from said fuel elements and from coolant owingthereover is provided.

The sleeving means preferably comprise graphite sleeves relatively thickso as to render them capable of being mechanically handled, since theywill normally be a structural part of fuel element assemblies and beremoved from the reactor and replaced by new assemblies on refuellingEach thermal barrier preferably comprises a further sleeve arranged toprovide a static gas gap between the graphite sleeve and itself. Sincethe thermal barrier sleeves will not normally be handled on re-fuelling,they may each be thin and may be of graphite or of impervioustemperature resistant material, for example zirconium metal.

The moderator is preferably a structure incorporating bricks of graphiteand having longitudinal fuel element channels comprising a rst grouporientated therein, the non-fuel element passages in the moderator forthe moderator coolant being provided by a second group of channelsbetween adjacent `longitudinal faces of the graphite bricks of themoderator structure. `The second group of channels are preferablyprovided by crenellating said adjacent faces, or alternatively may beprovided by chamfering the longitudinal corners of said adjacent faces.

A constructional example embodying the invention will now be describedwith reference to the accompanying FIGURE 1 is a diagrammaticfragmentary side View in section,

FIGURE 2 is a fragmentary perspective View,

FIGURE 3 is a diagrammatic fragmentary side view in section, and

FIGURE 4 is a fragmentary plane View.

FIGURES 2, 3 and 4 are drawn to larger scales than that of FIGURE l.

Referring to FIGURE 1 of the drawings, there is illustrateddiagrammatically a nuclear reactor having a pressure vessel consistingof` a top dome 101, having wall means defining an upper space 125, agenerally cylindrical portion 102, and a bottom dome 103 having wallmeans defining a lower space 126. The vessel 100 `contains a coreconsisting of a moderator 108 and graphite reflector pierced by a firstgroup of vertical channels 109 (two only of which are shown for the sakeof clarity) the majority intended for fuel elements andthe remainder forcontrol rods, shut-.olf devices, flux scanning gear, `graphite samplingequipment, etc. Above the core with clearance to leave a space 145 isdisposed a neutron shield 111 pierced by channel extensions formed bytubes 115 aligned and communicating with the first group of channels109, the tubes communicating through apertures 144 with a hot box orheader vessel 106 and with standpipes 116 which pierce the top dome 101and serve for access to the first group of channels 109 for refuellingand other purposes and are normally sealed by shield plugs (not shown).The` reector 110 and neutron shield 111 are surrounded by a thermalshield 112 spaced from the cylindrical wall of the portion `102 of thepressure vessel to provide a passage 113 therebetween. Components 108,110 and neutron shield 111 are supported by a diagrid 114 itselfsupported by brackets on the interior of the dome 103 of the pressurevessel, the load on the brackets being transferred through the shell ofthe pressure vessel to foundations 146.

The neutron shield is shown more particularly in FIG- URE 2 and isformed from columns of graphite bricks 147 interlayered with boronplates 148, the tubes 115 penetrating the bricks 147 and plates 148 andthe latter two components being provided with registering recesses 149which when the bricks and plates are assembled to form the completeneutron shield 111 form a third group of channels or passages 150extending from the top of the neutron shield 111 to the space 145between the neutron shield 111 and the reflector 110 of the core. Thecolumns forming the neutron shield 111 are each supported on a '"stool151wengaging awspigot 152 supported by the top of a registering columnof graphite bricks 153 which when assembled form the reflector 110 andmoderator 108. The spigot 152 engages the channel 109 in the respectivecolumn of bricks 153. The stool 151 has apertures 154 which communicatebetween the spaces 145 and the channel 109.

The header vessel 106 communicates via ducting 105 with one or more heatexchangers, one only being shown in FIGURE 1 for the sake of clarity anddesignated 117. A circulator 118 whose drive is contained in a housing137 is disposed within the shell of the heat exchanger 117 at the bottomthereof and return ducting 104 coaxial with the ducting 105 andproviding an annular passage 119 between the ducting 104, 105communicates with the interior of the pressure vessel. Coolant (forexample carbon dioxide) under pressure is circulated by the circulator118 and flow takes place along the annular passage 119 between ducting104 and 105, into the pressure vessel to cool the dome 101, then dividesinto two portions, one of which passes downwardly through the passage113 to cool the thermal shield 112 and the wall of the pressure vesselportion 102. The other portion passes downwardly through the channels150 in the neutron shie-ld 111 to cool it and from thence t-o the space145. Referring now to FIGURE 3, there is illustrated diagrammatically acore channel 109 and an assembly of fuel elements 121 disposed therein.Each fuel element 121 has a graphite sleeve 136 which is spaced from thewall of the channel 109 to provide an annular passage 133. Furthermore,there is a thermal insulating sleeve 134 disposed between Athe fuelmembers of the fuel element and the sleeve 136 so as to provide a staticgas gap 135 which serves to reduce heat transfer from the fuel membersand from coolant in heat transfer flow thereover, to the annular passage133. Furthermore, the core has channels of a second group extendingthrough it, one such channel being shown diagrammatically in FIGURE 3and designated 120. The channels of the second group are convenientlyformed in adjacent vertical faces in those graphite bricks 153 which arepenetrated by the channels 109, by crenellations 122, as shown inFIGURES 2 and 4. The channels 120 formed between the bricks 153 presenta large surface area of graphite to coolant flowing therethrough whilekeeping the cross-sectional area of the passages relatively smallthereby economising in neutrons and avoiding the necessity for increasedenrichment of the nuclear fuel contained in the fuel elements 121.

It will now be appreciated that the portion of coolant which enters thespace 145 via the third group of channels 150 again divides into aportion which flows downwardly through the second group of channels 120thereby cooling the moderator 108 and reflector 110, and a portion whichflows downwardly through the annular passages 133, the latter portionserving partly to cool the moderator 108 and reflector 110 and partly tocool the graphite sleeves 136 which themselves have a moderatingfunction and are main structural members of the fuel elements 121. Thesetwo portions combine below the bottom reflector 110 in the lower space126 and also combine with the portion of coolant flowing down the annu--ilar passage 113 between the pressure vessel portion 102`- and thethermal shield 112. The total combined coolant then passes upwardlythrough the fuel elements 121 in passages defined by the interior of thethermal insulation sleeve 134 and by heat transfer from the fuel membersextracts nuclear heat. The combined coolant then passes upwardly throughthe channel extensions or tubes 115 into the header vessel 106, alongthe ducting to the heat exchanger 117 wherein it flows upwardlysuccessively through a superheater 138, an evaporator bank 139 and aneconomiser bank 140 whereby the heat in the coolant is exchanged toraise and superheat steam which can be employed in well-known manner todrive a turbo-generator (not shown) for producing electric power. Thecoolant after giving up its heat then flows downwardly in contact withthe wall of the heat exchanger 117' to cool it, and thence tothecirculator 118 for return VYto -the reactor pressure vessel. It will beappreciated that the coolant flow takes place in a closed system asdescribed.

A neutron shield plug 123, shown diagrammatically in FIGURE 3, isemployed to scatter neutrons which escape up the tube and cause them tobe absorbed in the neutron shield 111. The shield plug 123 may be forexample as disclosed in our U.S. application Ser. No. 821,- 493, filedJune 19, y1959, now U.S. Patent No. 3,090,742. A gas seal 124 isemployed to prevent coolant flow from the upper regions of the pressurevessel from passing between the tubes 115 and the channels therefor inthe neutron shield 111.

The fuel elements for each channel 109 are preferably interconnected asa train. Each train is connected to a neutron shield plug 123 so that ondischarge, removal is as a complete assembly. Each neutron shield plug123 is preferably also connected by a distance piece to a seal plug forthe respective standpipe 116, so that charge and discharge of fuelelements may be effected from the charge face (not shown, but situatedat the top of the standpipes 116) without the necessity for employing agrab having to operate within the standpipes 116. The fuel elements maybe for example as described in our U.S. application Ser. No. 787,430filed January 19, 1959, now U.S. Patent No. 3,128,235.

It has been found that by employing the portion of the coolant flowingin the annular passage 113 and by passing the second group of channelsand the passages 133 in the moderator 108 and reflector 110, thepassages 133 and channels 120 can be made smaller than they would haveto be to accept the whole coolant flow without an unacceptable pressuredrop across the moderator and reflector, this resulting in neutroneconomy and avoiding loss in pumping power. On the other hand, eventhough the passages 133 and channels 120 can be made smaller byemploying the crenellations 122 to form the channels 120 a comparativelylarge surface area of graphite is presented to the coolant for effectivecooling of the moderator 108 and reiiector 110. In a specific case, withan approximately equal division of coolant between on the one hand thepassage 113 and on the other hand the channels 120 and passages 133,optimum moderator and reflector cooling, neutron economy and pumpingpower economy is effected.

The apertures 154 in the stools 151 form fixed gags for coolant flowingthrough the passages 133 and thereby determine the fraction flowingtherethrough. It is desirable that the amount of coolant flowing in thepassages l133 be4 restricted so that in the event of any fracture ofsleeves 136 and 134 or any separation of these sleeves from adjacentsleeves in the train of fuel elements 121 the amount of coolant whichcan short-circuit and join the flow upwardly over the fuel members islimited and fuel members which are by-passed are not subjected to undueoverheating.

In an alternative construction, the apertures 154 in the stools 151 andthe gas seal 124 may be omitted and all the portion of coolant owingdown the channels `intheneutron shield 111"is then constrained to flow`downthe channels 120 in the moderator 108 and reflector toithe` outlettemperature of the `,coolant flowing over the l"fuel :members `and maybe `effected as described in t ourjsaid application Ser. No. 821,493.

` The coolaht entering the refiector and moderator for f1 dolwnwardtfiowtherethrough is desirably in all cases at `anwinitial ,temperature inexcess of that necessary for "overcoming (or reducingto within safelimits) Wigner `energy,problems.

Instead of adjacent verticalfacestof the graphite bricks 1153i beingcrenellated, adjacent vertical corners of the j 'bricks 153i tnay bechamfered, as `indicated by the dott and-dash lines 155 in FIGURE 4, toprovide the coolant *passages through the;` core. `effective inpresenting a large surface area of the graphite This is, however,` not`as `ltoithecoolant as the embodiment hereinbefore described, butftmay beefficient? enough for reactors in which `the Vheatlrating isInothigh andsimplicity of design is called Vfor.

` We claim:

" LMAigraphitemoderated nuclear reactor arrangement for reducing theeffect of a positive moderator temperature coefficient comprising agraphite moderator strucl `ture:defiriin`g a series of parallel-disposedchannels extending rightthroughsaid structure 'and divided into firstandtsecond groups with a channel of the first group disposed between`adjacent channels of the other group andttspaced"from` the channels ofthe other group by l moderator material, `a header vessel` adjacent toand `spaced `from `saidstructure and traversing the extending l waxesofsaid channels, reactor `fuel -in the channels of said first `group 1onlyand means defining coolant paths around the fuel, channel extensionsconnecting the header vessel *with the adjacent` ends of the channels ofthe said first group only, means for feeding reactor coolant to theendslof thechannels of the said second group adjacent the ,theaderxvessel tov fioW through the channels of the secondgroupwmeans;restricting coolant from flowing into the endsof the fuelcoolant paths nearest the header vessel,

, =Wall` means extending transversely across the axes `of ,j`saidchannels` and arranged to constrain coolant emerging jin; bulklfromthe ends of the channels of said second L group remote from saidheader vessel to enter the ends l ofthe channelsof said first groupremote from the head- `en vessel, and flow through the fuel coolantpaths in the t l"channelsof the first group in contra-direction to theflow `ofthe coolantin the channels of the second group, means t`,defining a flow path from the fuelycoolant paths to the l "interior of`said `header vessel, and means for withdrawing `coolant frorntheinterior of said header vessel.

t 2.",lA lgraphitemoderated nuclear reactor arrangement t t forreducingthe effect of a` positive moderator temperature adjacent to andspaced from said structure and traversing i f `theextended axes of thechannels of said first group, chan- 1 nelextensions connecting theadjacent ends of the channels l of said first group with said headervessel, nuclear fuel in v saidzchannels` andtmeans defining a coolantpath around "the :fuel,`means defining a second group ofparallel-disposed channels extending right through said structureeach`channelof saidsecond group being ldisposed between adja- `centichannels ofzsaid first group and spaced therefrom by ghaphite moderatormaterial, means for feeding .reactor i g coolant tokone end only ofeachof the channels of said jsecond group-to flow through the channels ofthe second l 1 groupfmeans; restricting coolant from -owing into theends of the fuel coolant paths nearest the header vessel, wall meansextending transversely across the axes of all of the channels andyar-ranged to constrain coolant emerging in bulk from the other end ofthe channels of said second group to enter the ends of the channels ofsaid first group remote from said header vessel to flow through the fuelcoolant paths in the channels of the first group and through the channelextensions to the header vessel, and means fol withdrawing coolant fromthe interior of said header vessel.

3. A lgraphite moderated nuclear reactor arrange-ment for reducing theeffect of a positive moderator temperature coefficient comprising apressure vessel, a graphite moderator structure disposed within thepressure vessel and defining a series of parallel-disposed channelsextending right through said structure and divided into first and secondgroups with a channel of one group disposed between adjacent channels.of the other and spaced there- `from by moderator material, a headervessel adjacent to and spaced from said structure and traversing theextended axes of said channels, reactor fuel in the channels of thefirstgroup only and means defining coolant paths around `thereactorfuel, channel extensions connecting the header vessel with the adjacentchannels of said first group only, a heat exchanger external thepressure vessel and having a coolant inlet and a coolant outlet, lafirst duct connecting said coolant outlet with the interior of thepressure vessel, a second duct connecting said coolant inlet with saidheader vessel, coolant circulating means .causing Va flow of reactorcoolant to enter said pressure vessel by way of said first duct and toleave said header vessel by way of said second duct, first Wall meansconstraining coolant entering said pressure vessel as aforesaid to enterthe ends of the channels of said second `group adjacent said headervessel to fiow through the channels of the second group, meansrestricting coolant from flowing into the ends of the fuel coolant pathsnearest the header vessel, second Wall means extending transverselyacross the axes of said channels and arranged to constrain coolantemerging in bulk from the ends of the channels of said second groupremote from said `header vessel to enter the adjacent ends of thechannels of said first group and to flow through the fuel coolant pathsin the channels of the first group in contra-direction to the flow ofthe coolant in the channels of the second group, and means defining afiow of coolant from the channels of the first group to the headervessel.

4. A graphite moderated nuclear reactor arrangement for reducing theeffect of a positive moderator temperature coefficient comprising apressure vessel, a graphite moderator structure disposed within thepressure vessel and defining a first group of parallel-disposed channelsextending right through the lmoderator structure, a header vesseladjacent to and spaced from said structure and traversing the extendedaxes of said channels, channel extensions connecting said header vesselwith the adjacent ends of said channels, nuclear fuel in said channelsand means defining coolant paths around the fuel, means defining asecond group of parallel-disposed channels extending right through saidstructure, each channel of said second group being disposed betweenadjacent channels of said first group and spaced therefrom lby moderatormaterial, a heat exchanger external the pressure vessel and having acoolant inlet and a coolant outlet, a first duct penetrating `saidpressure vessel in -t-he region of one end only of the channels of saidsecond group and connecting the interior of said pressure vessel withsaid coolant outlet, a second vduct connecting sai-d coolant inlet withsaid header vessel, coolant circulating means causing a ow of `reactorcoolant to enter said pressure vessel by way of said first duct and toleave said header vessel by way of said second duct, first wall meansconstraining coolant entering said pressure vessel as aforesaid to enterthe adjacent ends of the channels of said second group to fiow throughthe channels of the second group, means constraining coolant fromflowing into the ends of the fuel coolant paths nearest the .headervessel, second wall means extending transversely across the axes of saidchannels and arranged to constrain coolant emerging in bulk from thelother ends of the channels of said second group to enter the adjacentends of the channels of the first group to flow through the coolantpaths in the channels of the first group in contra-direction to the flowof the coolant in the channels of the second group, and means defining acoolant flow from the .channels of the first group through the channelextensions to the header vessel.

5. A nuclear reactor comprising a pressure vessel, an upright-disposedmoderator structure contained within the pressure vessel and definingupper and lower spaces therewith, means defining a series ofvertically-orientated channels extending through said structure anddivided into first and second groups wtih each channel of one groupdisposed between adjacent channels of the other, a thermal shieldenclosing the sides of the moderator structure to define an annular voidwith the shell of the pressure vessel and to connect said upper andlower spaces, a header vessel disposed in said upper space, channelextensions connecting said header vessel with the upper ends of thechannels of said first group, nuclear fuel in the channels of said firstgroup only, a heat exchanger external the pressure vessel and having acoolant inlet and a coolant outlet, a first duct connecting said upperspace with said coolant outlet, a second duct connecting said headervessel with said coolant inlet and coolant circulating means causing afiow of reactor coolant to enter said pressure vessel by way of saidfirst duct and to leave said pressure vessel by way of said v secondduct.

6. A graphite moderated nuclear reactor arrangement for reducing theeffect of a positive moderate temperature coefficient comprising apressure vessel, an uprightdisposed graphite moderator structurecontained Within the pressure vessel and defining upper and lower spacestherewith, means defining a series of vertically-orientated channelsextending right through said structure and divided into first and secondgroups with each channel of one group disposed between adjacent channelsof the other and spaced therefrom by moderator material, a thermalshield enclosing the sides of the moderator structure to define anannular void with the shell of the pressure vessel and to connect saidupper and lower spaces, a header vessel disposed in said upper space,channel extensions connecting said header vessel with the upper ends ofthe channels of said first group, nuclear fuel in the channels of saidfirst group only and means defining coolant paths around said nuclearfuel, a heat exchanger external the pressure vessel and having a coolantinlet and a coolant outlet, a first duct connecting said upper spacewith said coolant outlet, a second duct connecting said header vesselwith said coolant inlet, coolant circulating means causing a flow ofreactor coolant to enter said pressure vessel by way of said first ductand to leave said pressure vessel by way of said second duct, wall meansconstraining fluid to fiow from the header vessel into the adjacent endsof the channels of the second group to fiow therethrough, meansrestricting the coolant from fiowing into the ends of the fuel coolantpaths nearest the header vessel, and additional wall means constrainingcoolant emerging from the other ends of the channels of the second groupto enter the ends of the channels of the first group remote from theheader vessel and flow through the fuel coolant paths in the channels ofthe first group in contra-direction to the flow of the coolant in thechannels of the second group, the channel extensions defining a coolantfiow path from the channels of the first group to the header vessel.

7. A graphite moderated nuclear reactor arrangement for reducing theeffect of a positive moderator temperature coefficient comprising apressure vessel, an uprightdisposed graphite moderator structurecontained within the pressure vessel and defining upper and lower spacestherewith, means defining a series of vertically-orientated v channelsextending right through said structure and divided into first and secondgroups with each channel of one group disposed between adjacent channelsof the other and spaced therefrom by moderator material, a thermalshield enclosing the sides of the moderator structure to define anannular void with the shell of the pressure vessel and to connect saidupper and lower spaces, a header vessel disposed in said upper space,channel Vextensions connecting said header vessel with the upper ends ofthe channels of said first group, nuclear fuel in the channels of saidfirst group only and means defining coolant paths around said nuclearfuel, a heat exchanger external the pressure vessel and having a coolantinlet and a coolant outlet, a first duct connecting said upper spacewith said coolant outlet, a second d-uct disposed coaxially within saidfirst duct and connecting said header vessel with said coolant inlet,coolant circulating means causing a'fiow of reactor coolant to entersaid pressure vessel by way of said first duct and to leave saidpressure vessel hy way of said second duct, wall means constrainingffuid to fiow from the header vessel into the adjacent ends of thechannels of the second group to flow therethrough, means restricting thecoolant from flowing into the ends of the fuel coolant paths nearest theheader vessel, and additional wall means constraining coolant emergingfrom the other ends of the channels of the second group to enter theends of the channels of the first group remote from the header vesseland flow through the fuel coolant paths in the channels of the firstgroup in contra-direction to the fiow of the coolant in the channels ofthe second group, the channel extensions defining a coolant flow pathfrom the channels of the first group to the header vessel.

8. A graphite moderated nuclear reactor arrangement for reducing theeffect of a positive moderate temperature coefcient comprising agraphite moderate structure composed of close-packed blocks of solidmoderate material and defining a first group of parallel-disposedchannels extending right through the moderate structure, each blockhaving a central passage defining part of one of the channels of saidfirst group, a header vessel adjacent to and spaced from said structureand traversing the extended axes of the channels of said first group,channel extensions connecting the adjacent ends of the channels of thefirst group with said header vessel, nuclear fuel in said channels andmeans defining a coolant path around the fuel, means defining a secondgroup of parallel-disposed channels extendingright through saidstructure, each of said blocks defining at least one side face having arecess defining part of one of the channels of the second group, eachchannel of said second group being disposed between adjacent channels ofsaid first group and spaced therefrom by graphite moderate material,means for feeding reactor coolant to one end only of each of thechannels of the second group to flow through the channels of the secondgroup, means restricting coolant from flowing into the ends of the fuelcoolant paths nearest the header vessel, wall means extendingtransversely across the axes of all of the channels and arranged toconstrain coolant emerging in'bulk from the other end of the channels ofsaid second group to enter the ends of the channels of said first groupremote from said header vessel to fiow through the fuel coolant paths inthe channels of the first group and through the channel extensions tothe header vessel, and means Yfor withdrawing coolant from the interiorof said header vessel.

9. A graphite moderated nuclear reactor arrangement for reducing theeffect of a positive moderative temperature coefiicient comprising agraphite moderate structure defining a first group of parallel-disposedchannels extending right through the moderate structure, a header vesseladjacent to and spaced from said structure and traversing the extendedaxes of the channels of said first group, channel extensions connectingthe adjacent ends of the `channels of the first group with said headervessel,

i nuclearreactor fuel` in said `channels and means defining a coolantpath around the fuel, the fuel coolant path defining?meanscomprising asleeve member of solid moderator material in `each channel enclosing thenuclear fuel anddefining an annular passage with the wall of saidchannel so as to divide flow through said channel into a fuel coolantstream and a moderator coolant stream and i insulating means forimpeding transfer of heat between the `two streams, means defining asecond group of paralt lel-,disposed channels extending right throughsaid strucfeeding `reactor coolant to one end only of each of thechannels of said second `group to ow through the channels ofgthe secondgroup, means restricting coolant from flowing into the ends of the fuelcoolant paths nearest thei` header vessel, wall means extendingtransversely acrosswthe axes of all of the channels and arranged to tconstrain coolant emerging in bulk from the other end of i the `channelsof said first group to enter the ends of the channels `of the secondgroup remote from said header vessellzto ow ,through the fuel coolantpaths as coolant streams `in the channels of the first group and throughthe channel extensions to the header vessel, and means for withdrawingcoolant from the interior of said header vessel.

10. A nuclear reactor as claimed in claim 7 and provided with a neutronshield structure disposed between said moderator structure and saidheader vessel, said neutron shield structure being penetrated by saidchannel extensions and also being penetrated by a third group ofchannels defining a plurality of flow paths for coolant flowing fromsaid upper space to the channels of said second group.

References Cited by the Examiner UNITED STATES PATENTS 2,774,730 12/1956Young 176--58 2,780,596Y n2/1957 Anderson aaff'176-58 3,090,742 5/ 1963Fawcett 176-59 OTHER REFERENCES Proceedings of First Geneva Conferenceon Peaceful Uses of Atomic Energy, 1955, published by U,N., vol. 2,pages 342-347.

L. DEWAYNE RUTLEDGE, Primary Examiner.

1. A GRAPHITE MODERATED NUCLEAR REACTOR ARRANGEMENT FOR REDUCING THEEFFECT OF A POSITIVE MODERATOR TEMPERATURE COEFFICIENT COMPRISING AGRAPHITE MODERATOR STRUCTURE DEFINING A SERIES OF PARALLEL-DISPOSEDCHANNELS EXTENDING RIGHT THROUGH SAID STRUCTURE AND DIVIDED INTO FIRSTAND SECOND GROUPS WITH A CHANNEL OF THE FIRST GROUP DISPOSED BETWEENADJACENT CHANNELS OF THE OTHER GROUP AND SPACED FROM THE CHANNELS OF THEOTHER GROUP BY MODERATOR MATERIAL, A HEADER VESSEL ADJACENT TO ANDSPACED FROM SAID STRUCTURE AND TRAVERSING THE EXTENDING AXES OF SAIDCHANNELS, REACTOR FUEL IN THE CHANNELS OF SAID FIRST GROUP ONLY ANDMEANS DEFINING COOLANT PATHS AROUND THE FUEL, CHANNEL EXTENSIONSCONNECTING THE HEADER VESSEL WITH THE ADJACENT ENDS OF THE CHANNELS OFTHE SAID FIRST GROUP ONLY, MEANS FOR FEEDING REACTOR COOLANT TO THE ENDSOF THE CHANNELS OF THE SAID SECOND GROUP ADJACENT THE HEADER VESSEL TOFLOW THROUGH THE CHANNELS OF THE SECOND GROUP, MEANS RESTRICTING COOLANTFROM FLOWING INTO THE ENDS OF THE FUEL COOLANT PATHS NEAREST THE HEADERVESSEL, WALL MEANS EXTENDING TRANSVERSELY ACROSS THE AXES OF SAIDCHANNELS AND ARRANGED TO CONSTRAIN COOLANT EMERGING IN BULK FROM THEENDS OF THE CHANNELS OF SAID SECOND GROUP REMOTE FROM SAID HEADER VESSELTO ENTER THE ENDS OF THE CHANNELS OF SAID FIRST GROUP REMOTE FROM THEHEADER VESSEL, AND FLOW THROUGH THE FUEL COOLANT PATHS IN THE CHANNELSOF THE FIRST GROUP IN CONTRA-DIRECTION TO THE FLOW OF THE COOLANT IN THECHANNELS OF THE SECOND GROUP, MEANS DEFINING A FLOW PATH FROM THE FUELCOOLANT PATHS TO THE INTERIOR OF SAID HEADER VESSEL, AND MEANS FORWITHDRAWING COOLANT FROM THE INTERIOR OF SAID HEADER VESSEL.